▎ 摘　　要

The possible detection of C-24, a planar graphene that was recently reported to be in several planetary nebulae by Garcia-Hernandez et al., inspires us to explore whether and how much graphene could exist in the interstellar medium (ISM) and how it would reveal its presence through its ultraviolet (UV) extinction and infrared (IR) emission. In principle, interstellar graphene could arise from the photochemical processing of polycyclic aromatic hydrocarbon (PAH) molecules, which are abundant in the ISM, due to the complete loss of their hydrogen atoms, and/or from graphite, which is thought to be a major dust species in the ISM, via fragmentation caused by grain-grain collisional shattering. Both quantum-chemical computations and laboratory experiments have shown that the exciton-dominated electronic transitions in graphene cause a strong absorption band near 2755 angstrom. We calculate the UV absorption of graphene and place an upper limit of similar to 5 ppm of C/H (i.e., similar to 1.9% of the total interstellar C) on the interstellar graphene abundance. We also model the stochastic heating of graphene C-24 in the ISM, excited by single starlight photons of the interstellar radiation field and calculate its IR emission spectra. We also derive the abundance of graphene in the ISM to be <5 ppm of C/H by comparing the model emission spectra with that observed in the ISM.